Known variable voltage gain amplifiers include operational amplifiers of both single- and multi-stage designs. These conventional amplifiers often suffer from such performance limitations as limited dynamic range, distortion in the presence of heavy output loads, and a trade-off between the voltage gain available and the bandwidth over which the amplifier provides a substantially flat voltage gain profile.
FIG. 1A is a schematic diagram of a conventional, single-stage, variable gain amplifier (VGA) 11 comprised of both n-channel and p-channel metal oxide semiconductor field effect transistors (MOSFETs), in accordance with conventional CMOS technology. The amplifier includes a pair of NMOS transistors N12 and N13 for receiving a differential input signal represented by the difference between input signals v.sub.G1 and v.sub.G2. The input transistors are biased by a constant current source 14, which is typically implemented with a current mirror configuration. Input transistors N12 and N13 are actively loaded with a pair of PMOS transistors P15 and P16. The drains of P15 and P16 are respectively coupled to the drains of input transistors N12 and N13 to provide a variable load in response to a voltage control signal (v.sub.c) applied to the gates of each of the load transistors. A variable resistor R19, coupled between the drains of the input transistors, operates as a bandwidth limited gain control to adjust the gain while varying the bandwidth of the amplified differential output signals which are generated at opposite ends of the resistor.
FIG. 1B illustrates another conventional, single-stage VGA design 21. This design is similar to the design shown in FIG. 1A, except that this design employs a pair of variable resistors R22 and R23 instead of voltage-controlled load transistors. Also, there is no variable resistor coupled between the drains of the input transistors.
A basic problem with conventional amplifiers of the types shown in FIGS. 1A and 1B is that voltage gain is substantially inversely proportional to the bandwidth of the amplifier. In fact, the gain-bandwidth product in such amplifiers remains nearly constant in the operating range of the amplifier. Thus, an increase in gain decreases the bandwidth, and vice-versa. These conventional amplifiers lack the ability to provide a variable gain, while maintaining a substantially constant bandwidth. The amplifier illustrated in FIG. 1B also does not provide a constant common-mode output.